Tar sands, also referred to as oil sands and bituminous sands, contain a heavy oil usually referred to as bitumen.
There are tar sand deposits, in the Athabasca region of Alberta, which are today being commercially exploited. In connection with these operations, the tar sand is first mined and the bitumen is then extracted from the mined tar sand by a process called the hot water process. The extracted bitumen is subsequently upgraded by refinerytype processing, to produce synthetic crude.
The tar sand is a mixture of sand grains, connate water, fine minerals of the particle size of clay, and bitumen. It is commonly believed that the connate water envelopes the grains of sand, the fine solids are distributed in the water sheaths, and the bitumen is trapped in the interstitial spaces between the water-sheathed grains.
The hot water process is now well described in the patent and technical literature. A schematic of the circuit is shown in FIG. 3.
In broad summary, this process comprises first conditioning the tar sand, to make it amenable to flotation-sedimentation separation of the bitumen from the solids. Conditioning involves feeding mined tar sand, hot water (180.degree. F.), an alkaline process aid (usually NaOH), and steam into a rotating horizontal drum, wherein the ingredients are agitated together. Typically, the amounts of reagents added are in the following proportions:
tar sand--3250 tons PA1 hot water--610 tons PA1 NaOH--4 tons (20% NaOH) PA1 14.44%--bitumen PA1 0.36%--water PA1 85.2%--total solids PA1 7.56%--bitumen PA1 0.5%--water PA1 91.84%--total solids. PA1 adjust the rate of NaOH addition; or PA1 adjust the rate of water addition to the conditioning or flooding steps; or PA1 blend some better quality tar sand in with the lean tar sand, to provide a blended feed; or PA1 vary the residence time or temperature in the conditioning drum. PA1 withdrawing a sample from the middlings dragstream and measuring the sample viscosity with an appropriate instrument; or PA1 lowering a sampler into the middlings, taking a grab sample, and measuring the sample viscosity with an appropriate instrument; or PA1 applying density measurements to either of the foregoing samples and assuming that the viscosity varies proportionately with the density. PA1 (1) that the viscosity varies strikingly at various depths in the middlings in the PSV; PA1 (2) that while the in situ-measured viscosity in the PSV may vary significantly, the density of the middlings when measured in connection with grab samples may vary very little--therefore there does not appear to be a useful correlation between the two that may be relied on; and PA1 (3) that the viscosity measurements obtained in situ vary significantly from those obtained by taking grab samples at the same depth in the PSV and measuring the viscosity of the grab samples in a conventional instrument external of the PSV.
Enough steam is added to ensure an exit temperature of the mixture from the drum of about 180.degree. F. The residence time in the drum is typically about 4 minutes.
During conditioning, the mined tar sand (in which the bitumen, connate water and solids are tightly bound together) is converted into an aqueous slurry of porridge-like consistency, wherein the components are in loose association.
The slurry leaving the drum is screened, to remove oversize material, and then flooded or diluted with additional hot water. The diluted slurry typically comprises 7% by weight bitumen, 43% water, and 50% solids. Its temperature is typically 160.degree.-180.degree. F.
The diluted slurry then is transferred to the primary separation step, wherein it is temporarily retained in a large separation vessel having a cylindrical upper section and conical lower section. (This vessel is hereafter referred to as the "PSV"--for `primary separation vessel`.) The vessel is similar to a thickener and has a rake system in its lower end, to assist in discharging the sand bed which accumulates there. The slurry is retained in the PSV for about 45 minutes in a quiescent condition.
During this interval, air bubbles, incorporated into the dilute slurry during conditioning, attach themselves to the bitumen, which is in the form of flecks or globules. Most of the aerated globules are buoyant and they rise through the slurry, to collect at the upper surface in the form of a froth. This froth is referred to as primary froth.
Most of the coarse solids, primarily being sand particles, sink through the slurry, are concentrated in the conical bottom end of the vessel, and are discharged through a bottom outlet. This stream is discarded as tailings (known as the `primary tailings`).
Not all of the bitumen becomes sufficiently aerated so as to rise and join the primary froth. Some of this non-buoyant bitumen is lost with the primary tailings. Most of it, together with a large part of the fines, collects in the mid-section of the PSV. This aqueous mixture is termed "middlings".
A dragstream of the middlings is withdrawn from the vessel and is fed into subaerated flotation cells, wherein secondary separation is practised. Here the middlings are subjected to vigorous agitation and aeration. Bitumen froth, termed "secondary froth", is produced.
Typically, the primary and secondary froths have the following compositions:
______________________________________ Primary (% by weight) Secondary (% by weight) ______________________________________ Bitumen 66.4 23.8 Solids 8.9 17.5 Water 24.7 58.7 ______________________________________
It will be noted that the secondary froth is considerably more contaminated with water and solids than the primary froth. One seeks to minimize this contamination, as the froth stream requires downstream treatment, to remove solids and water, before it can be fed to the upgrading process.
It is therefore desirable to operate the process so that as much of the bitumen as possible reports to the primary froth.
In summary then, the contents of the PSV may be described as existing in the form of three sequential layers. At the base, one has the tailings--this is primarily sand with some water and a minor amount of bitumen entrained therein. Above this layer, one has the middlings--this is water containing fines and insufficiently buoyant bitumen. But passing downwardly through the middlings are many coarse sand particles and rising through the layer are some buoyant bitumen globules. And at the top, one has the froth.
Of particular interest are the well-aerated bitumen globules, which should rise and form the primary froth, which is the main commercial product of the process. These globules must make their way up through the middlings.
If the middlings are too viscous, the well-aerated bitumen globules may fail to achieve the needed upward velocity, and may end up being discharged with the primary tailings or being withdrawn with middlings for treatment in the secondary separation circuit. If the globules exit with the primary tailings, they are lost entirely from the process. If they are removed to secondary recovery, they will be recovered in the form of poor quality froth.
At this point, it is appropriate to point out: (1) that the nature of the tar sand feed is variable; and (2) that the capability of the hot water process to extract the contained bitumen is significantly affected by the nature of the tar sand feed.
More particularly, the tar sand may contain a relatively high content of bitumen and a relatively-low content of fines. This type of feed is referred to as "rich" tar sand. Alternatively, the tar sand may be relatively low in bitumen and high in fines. Such a feed is referred to as "lean" tar sand.
Typically, a "rich" tar sand can have a composition as follows:
Typically, a "lean" tar sand can have a composition as follows:
The percentage fine solids (-44.mu. solids in the total solids) can range from 5% for rich tar sands to as high as 25% for some lean tar sands.
In general, the rich tar sand feeds yield high primary froth recoveries. The lean feeds give low primary froth recoveries. Stated otherwise, the lean feeds are difficult to process with the hot water extraction procedure; they do not contain much bitumen and such bitumen as they do contain is difficult to extract.
This is partly because the lean feeds contain many fines, which interfere with the flotation-sedimentation separation taking place in the middlings layer of the PSV. In addition, the flecks or globules of bitumen which appear in the PSV middlings, when lean tar sand is the feed, are minute compared to the globules that are there when the tar sand feed is rich. These minute flecks do not rise as readily as the larger flecks.
If the fines content in the middlings becomes high, the flotation mechanism can literally become inoperative. There is so little primary froth being produced that the process performance is unacceptable. In this instance, the contents of the PSV may have to be jettisoned and the process started up again.
There are a number of courses of action open to the operator by which he may adjust and alleviate undesirable process conditions in the PSV arising from the nature of the tar sand feed. For example, he can:
A crucial matter, though, is to know when to make these adjustments and to what extent the adjustment should be made. This requires that a process parameter be monitored, which parameter gives the operator a useful guide on which to base the adjustments.
It has heretofore been broadly taught in the prior art that the viscosity of the middlings can be monitored and maintained within staged ranges, to optimize the primary bitumen froth recovery from the PSV. This teaching appears in Canadian Pat. No. 889,823, filed by Graybill et al. Also of interest are Canadian Pat. Nos. 889,825 and 841,581.
However, in accordance with conventional practise, the viscosity has been monitored in one of the following ways:
Now, there are certain shortcomings associated with these prior art practises.
If one samples the middlings dragstream, one must assume that this sample--taken at one level of the PSV (there is usually only a single outlet in the PSV wall)--is representative of the entire column of PSV middlings.
When one attempts to measure the viscosity of this sample, one is dealing with a mixture of sand, oil, clay, and water. The sand and oil begin to settle and rise instantaneously. In addition, the mixture is not static. It is impossible to duplicate the flow and turbulence conditions which exist within the PSV.
Perhaps for these reasons, the industry has moved toward measuring the density of the sample and assuming that the trend of viscosity will follow the trend of density.